Marmot Helium 15 down. I own two of them, both around 8 to 10 years old. Not sure it's the "best" bag, but they've sure worked great for me. Light weight, pack down really small and are accurate to the temp rating, at least on a good inflatable insulated pad. The shells have lasted really well over the years, but I don't abuse them and they are usually inside a bivy sack as well. The down has kept its loft, I hang them in the closet in the off season and do not wash them unless I have to, the. I use some sort of tech wash used for down in a large front lid washer, and throw a couple tennis balls in my large dryer. I've probably only washed them a couple of times. They are holding up well. I think I payed $350+ for hem back when I bought them.
